Liquid crystal display devices (LCD) have been produced by filling a liquid crystal composition in a sealed cell formed between two substrates provided with transparent electrodes. LCD have been practically used in various modes such as twist nematic (TN) mode, super twist nematic (STN) mode, and thin-film transistor (TFT) mode since their electric power consumption is small compared with CRT (cathode ray tube display) and since downsizing and weight-lightening are possible. Among the modes, active matrix LCD (AM-LCD) such as TFT have been watched as a prospective winner of flat display accompanied with the progress in actualization of colored display and fine picture image in AM-LCD.
Following characteristics are required of the liquid crystal compositions for AM-LCD:
1) Proper optical anisotropy (.DELTA.n) can be produced depending on cell thickness. PA1 2) Voltage holding ratio (VHR) is high in order to maintain a high contrast of LCD. PA1 3) Proper threshold voltage (V.sub.th) can be actualized depending on driving circuit. PA1 4) Range of nematic liquid crystal phase is wide depending on application environment (wide range).
AM-LCD have adopted for their driving a TN display mode in which the molecular orientation of a liquid crystal composition filled between an upper and a lower substrate is twisted by 90.degree.. TN display mode has a problem that the coloring of liquid crystal cell is caused due to interference when voltage is not applied. In order to avoid the coloring and to obtain an optimum contrast, the product (.DELTA.n.multidot.d) of .DELTA.n and cell thickness d (.mu.m) must be established to a certain value, for example, to a 0.5 .mu.m. Since such restriction exists, a main current of .DELTA.n of liquid crystal compositions for TFT currently used in practice is generally about 0.07 to about 0.11 and particularly 0.08 to 0.10 for 1st. Min. system.
In recent years, the development of LCD with a purpose of providing LCD for portable type is active as seen from the appearance of small size and light weight notebook type personal computers. With respect to such portable LCD, the reduction of production cost is desired in addition to further weight-lightening and downsizing whereas there are many restrictions in the aspect of driving electric power. As a means to meet such demand, liquid crystal materials of small electric power consumption and low V.sub.th are called to mind, and the development of such materials is expected.
Accompanied with the appearance of the portables, exploration for LCD intended for outdoor use has come to be attempted. In order to withstand outdoor use, liquid crystal compositions are considered to be necessary to exhibit a nematic phase even in a range beyond the temperature range of application environment. From such viewpoint, liquid crystal compositions having a nematic-isotropic phase transition temperature (clearing point T.sub.NI) of 60.degree. C. or higher and smectic-nematic phase transition temperature (T.sub.SN) of lower than -20.degree. C. have become a main stream of liquid crystal compositions for TFT currently used in practice.
In order to respond to such demand, various types of liquid crystalline compounds and liquid crystal compositions containing the compounds have been developed. For instance, Laid-open Japanese Patent Publication No. Hei 2-233626 has disclosed in its Application Example 2 a composition comprising 15% by weight of a trifluoro compound having a comparatively large dielectric anisotropy (.DELTA..epsilon.) and 85% by weight of a difluoro compound. However, this composition has defects that V.sub.th is high, the miscibility of the components in the composition are deteriorated particularly at low temperatures, and nematic phase range is narrow.
WO 94/03558 has disclosed compositions comprising a trifluoro compound and a difluoro compound. However, the compositions disclosed in its Examples 1 and 2 have such a low clearing point as lower than 50.degree. C. and have a .DELTA.n of lower than 0.06, and thus the compositions are short of practical utility.
Liquid crystal compositions are diligently being studied depending on various purposes as discussed above, but still they are not sufficient. Thus, it is a present situation that new improvements are all the time demanded.